A fable by Aesop.
A boy is tending a flock of sheep on a hill and gets bored. To alleviate his boredom, he cries "Wolf! Wolf!" This gets the attention of the local villagers, who run up the hill to help drive away the wolf. Initially, they believe the boy's tall tales of driving off the wolf single-handedly, becoming quite angry with the boy when they discover that there was in fact no lupine threat. They sternly admonish him not to cry wolf if there is in fact no wolf, and no danger.
The next day, the boy cries "Wolf! Wolf!" again, and again the villagers run up the hillside to find, again, no wolf. The villagers again tell the boy not to cry wolf.
The following day, the boy does it again, but this time there really is a wolf. However, the villagers don't heed his cry for help, and the wolf kills the flock (and in some versions of the tale, the boy as well).
One moral of the story is that a liar will never be believed ... even should he tell the truth.
A more twisted moral, and my personal favorite, is illustrated by the following snippet of dialogue between Garak and Dr. Bashir from the Deep Space Nine episode entitled "Improbable Cause".
Garak: "Are you sure that's the moral of the story, Doctor?"
Bashir: "Of course. What else could it possibly be?"
Garak (leaving infirmary, extremely self-satisfied): "That you should never tell the same lie twice."